1. Field Of The Invention
The present invention relates to variable electrical devices, and more particularly, to variable electrical devices having a rotational variable control that can be driven by a motor.
2. Discussion Of The Prior Art
In general, the rotational variable control for a variable electrical device is rotated by an external force. A motor drive unit can be used to create the external force. Thus, an electrical signal can be used to actuate the motor drive unit to adjust the position of a rotational variable control of a variable electrical device. Thus, small voltage signals can be used to control large voltages in high current situations.
A prior art apparatus 100 that adjusts the position of a rotational variable control shaft 102 of a variable electrical device using a motor drive unit 104 is shown in FIG. 1. The variable electrical device in FIG. 1 is a variable transformer 106. By applying an electric signal to the motor drive unit 104, the rotational position of the rotational variable control shaft 102 can be adjusted so as to control the value of voltage transformation by the variable transformer 106.
The variable transformer 106 shown in FIG. 1 includes a toroidal coil 108 in which the value of voltage transformations are changed by movement of a brush 110 along a commutator (not shown). The brush 110 is attached to an arm 112. The rotational variable control shaft 102 is attached to the arm 112 adjacent to the axial center of the toroidal coil 108. As a result of the rotational variable control shaft 102 being rotated, the brush 110 is rotated about the commutator (not shown) so as to change the value of voltage transformation by the variable transformer 106.
As shown in FIG. 1, the motor drive unit 104 is mounted on an upper bearing support plate 114. An upper bearing 116 is mounted in the upper bearing support plate 114 to rotationally support an upper portion of the rotational variable control shaft 102 for the variable transformer 106. A lower bearing 117 is mounted in a lower bearing support plate 118 to provide rotational support for a lower portion of the rotational variable control shaft 102 for the variable transformer 106. The upper bearing support plate 114 and lower bearing support plate 118 are separated by stanchions 120. The variable transformer 106 is positioned between the stanchions 120 and also between the upper bearing support plate 114 and the lower bearing support plate 118.
The rotational variable control shaft 102 of the variable transformer 106 is driven by the motor drive unit 104 using either a belt or gear arrangement in both a clockwise or counter-clockwise rotational movement. FIG. 1 illustrates a gear arrangement for driving the rotational variable control shaft 102. As shown in FIG. 1, a motor gear 122 on the output shaft 124 of the motor drive unit 104 meshes with a drive gear 126 on the rotational variable control shaft 102.
The rotational movement of the rotational variable control shaft 102 in either direction is limited by the activation of limit switches 128a and 128b that are positioned on the top of the upper bearing support plate 114. FIG. 2 is a top view of a prior art motor driven variable transformer. As shown in FIG. 2, cams 129a and 129b cause the limit switches 128a and 128b to turn off power to the motor drive unit 104 at the ends of the rotational range of the rotational variable control shaft 102. The cams 129a and 129b are mounted axially on the rotational variable control shaft 102 above the upper bearing support plate 114. As the rotational variable control shaft 102 rotates, the cams 129a and 129b travel about such that they can respectively activate limit switches 128a and 128b to prevent further rotation at the ends of the rotational range of the rotational variable control shaft 102.
As shown in FIG. 1, the output shaft 124 of the motor drive unit 104 in FIG. 1 is perpendicular to the upper bearing support plate 114. The variable transformer 106 is attached to the lower bearing plate 118. The upper bearing support plate 114 is rigidly held in relation to the variable transformer by the use of bolts 130 through the stanchions 120 to the lower bearing support plate 118. The upper bearing support plate 114 is mounted so that the rotational axis of the rotational variable control shaft 102 is parallel to the output shaft 124 of the motor drive output unit 104.
The prior art apparatus 100 for adjusting the rotational position of a rotational variable control shaft 102, as discussed above, requires that rotational variable control shaft 102 to be aligned with the upper bearing 116 and the lower bearings 117a and 117b. This alignment through the upper bearing 116 and lowering bearings 117a and 117b maintains the gear 126 in axial alignment with the gear 122 mounted to the motor drive output shaft 124 of the motor drive output unit 104. The arm 112 of the variable transformer 106 is held in a consistent axial relationship with the toroidal coil 108 by bearings 117a and 117b, so that the brush 110 applies a constant pressure throughout the entire travel range of the rotational variable control shaft 102. Further, the output shaft 124 of the motor drive unit 104 has to be aligned so as to be in parallel with the rotational variable control shaft 102.
The alignment requirements and use of gears or a belt and pulley system to transmit rotational motion for a prior art motor driven variable transformer, such as shown in FIG. 1, increase the complexity of manufacturing and the overall unit size. The implementation of a simpler and more efficient direct drive method is desirable. The bearing support plates and stanchions may, due to outside forces, become misaligned causing the rotational variable control shaft 102 of the prior art motor driven variable transformer to be in misalignment. This misalignment will degrade the operating capability of the prior art motor driven variable transformer due to binding which hampers smooth and consistent control in driving the rotational variable control shaft. Such binding can also cause bearing failure that may result in an inoperable device.